Biothermal power source for implantable devices

ABSTRACT

An implantable, rechargeable assembly comprised of an implantable device disposed within a living organism, an electrical storage device connected to the implantable device, and a thermoelectric charging assembly operatively connected to the electrical storage device. The thermoelectric charging assembly has devices for transferring thermal energy between the living organism and a thermoelectric module, for generating an electrical current from the thermal energy, for charging the electrical storage device with the electrical current, for determining the extent to which the electrical storage device is being charged with the electrical current, and for generating a signal whenever the extent to which the electrical storage device is being charged with the electrical current falls below a specified value.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional of co-pending patent application U.S.Ser. No. 10/694,548, filed on Oct. 27, 2003 which is acontinuation-in-part of patent application U.S. Ser. No. 10/098,821,filed on Mar. 15, 2002 now U.S. Pat. No. 6,640,137. The content of eachof the aforementioned patent applications is hereby incorporated byreference into this specification.

FIELD OF THE INVENTION

A device for providing a permanent source of energy comprised of athermoelectric module for creating electrical energy created by atemperature gradient in the body.

BACKGROUND OF THE INVENTION

Implantable medical devices (such as, for example, cardiac assistdevices, drug infusion pumps, and pain management devices) all requireelectrical power to carry out functions such as pacing the heart,delivering a drug, or stimulating nerves. These devices also requireelectrical power for basic control functions and for communicating withother devices and with external controllers.

The use of these and other active medical implantable devices is growingin popularity as new technology enables further miniaturization and asthe basic understanding of disease grows. An example of this is therecent expansion of electronic sensing and stimulation technology toapplications in deep brain stimulation (DBS), which shares much of thetechnology developed for cardiac pacing systems and is providing reliefto people suffering from Parkinson's disease and epilepsy. The expansionof applications for implantable medical devices will only accelerate inthe future.

One of the early limitations to the implantation of medical electronicswas the power source itself, and much development work has been doneover the past forty years to improve the reliability and longevity ofbattery sources and to reduce the power demands of the pacemakercircuits themselves. As a result, current pacemaker batteries lastseveral years in many applications.

In spite of these improvements in pacemaker and battery design, andespecially in the case of other implantable devices that put heavydemands on their power source, one of the primary reasons for surgicalremoval of cardiac pacemakers, drug delivery pumps, and other implanteddevices is battery lifetime. The need for surgical removal of an entireimplanted device is often the result of the need to integrate thebattery into the primary device case in order to eliminate corrosionthereof and the adverse health effects of leakage.

Some attempts have been made to provide a renewable power source bymeans of applying external power, primarily by inductive coupling ofradio frequency (RF) energy to an internal, implanted antenna. However,the use of this technique is inconsistent with the use of magneticresonance imaging (MRI), so patients with a radio-frequency rechargeableimplanted device will be unable to have MRI diagnoses of potentiallyserious conditions. Additionally, radio-frequency induction oftenrequires a patient to regularly set aside time for the rechargingprocess; this is often inconvenient and frequently causesnon-compliance.

The implantable devices that require such power sources are well knownin the art; their power sources are often used to provide power forsensing, control, tissue stimulation, drug dispensing, externalcommunication, and other necessary functions.

Some of the prior art improvements in such power sources are discussedbelow by reference to several United States patents; the entiredisclosure of each of these United States patents is hereby incorporatedby reference into this specification.

U.S. Pat. No. 6,108,579 illustrates some of the recently developedtechnology relating to such power sources. This patent discloses abattery-monitoring apparatus and method for programming of cardiacstimulating devices; the patent specifically discusses methods fortracking power usage, monitoring battery state, and displaying theestimated remaining life of the battery power source.

By way of further illustration, U.S. Pat. No. 6,067,473 discloses animplantable medical device that uses audible sound communication toprovide warnings of low battery life; among the warnings provided arevoiced statements warning of battery depletion.

U.S. Pat. No. 5,957,956 discloses an implantable cardioverterdefibrillator (ICD) having a relatively small mass and a minimal rate ofpower consumption. Similarly, U.S. Pat. No. 5,827,326 discloses animplantable cardioverter defibrillator having a smaller energy storagecapacity.

U.S. Pat. No. 5,697,956 discloses an implantable stimulation devicehaving means for optimizing current drain. Similarly, U.S. Pat. No.5,522,856 discloses a pacemaker with improved shelf storage capacity.Both of these patents describe means for minimizing the powerrequirements of battery power sources.

Recent emphasis on the availability of magnetic resonance imaging (MRI)diagnoses for patients has also created a focus on the inappropriatenessof conducting such MRI procedures on patients who have implantabledevices, such as cardiac pacemakers, installed. The electrical leadsused in such implantable devices to both sense heart function andprovide electrical pulses to stimulate the heart also act as antennae inthe intense magnetic and radio frequency (RF) fields used in MRIprocedures; the inductively coupled radio frequency energy received bysuch “antennae” are often sufficient to damage or destroy the pacemakeritself, and/or to create unwanted pacing of the heart, and/or to ablateblood vessels, and/or to scar sensitive heart tissue at theelectrode/heart interface. Death of the patient may result, and hasresulted, from one or more of these phenomena.

More recent developments in MRI technology have created the opportunityfor magnetic resonance angiography (MRA), which is the use of MRItechniques that are focused on cardiac structures and function. Thisdirect use of MRI at the heart often further exacerbates the existingdifficulties in using MRI on pacemaker patients.

One solution to these problems is the subject of U.S. provisional patentapplication Ser. No. 60/269,817; the entire disclosure of such patentapplication is hereby incorporated by reference into this specification.The approach disclosed in this patent application is the use of fiberoptics in place of electrical leads for pacemakers and for otherimplantable devices. The devices of this patent application providemeans for transmission via MRI-proof optical fibers, and thenre-conversion from optical to electrical pulses at the heart. In orderto serve the relatively higher power demands of this optical solution tothe MRI problem, either battery size must be substantially increased, orpacemaker installed life must be substantially shortened, or a means forrecharging the pacemaker power source must be utilized.

By way of further illustration, U.S. Pat. No. 4,014,346 discloses ahermetically sealed cardiac pacer system and recharging systemtherefore. The approach taken in this patent is to use inductivecoupling of external energy to recharge an internal battery. This andother similar approaches would help resolve the battery life issuesdiscussed above, were it not for this critical issue of MRI diagnoses;the very presence of an element that can accept externally-providedradio frequency energy makes MRI compatibility for this and similardevices impossible.

Thus, there is a need to provide a power supply means for periodicrecharging of an implantable device that is not susceptible to thedeleterious effects of MRI and other forms of electromagneticinterference. Further, there is a need to provide this capability in amanner that does not detract from the nominal performance of theimplantable device and to do so in a manner that is convenient for thepatient.

It is an object of this invention to provide such an improved powersupply.

SUMMARY OF THE INVENTION

In accordance with this invention, there is provided an implantable,rechargeable assembly comprised of an implantable device disposed withina living organism, an electrical storage device connected to saidimplantable device, and a thermoelectric charging assembly operativelyconnected to said electrical storage device, wherein said thermoelectriccharging assembly is comprised of means for transferring thermal energybetween said living organism and a thermoelectric module, means forgenerating an electrical current from said thermal energy, means forcharging said electrical storage device with said electrical current,means for determining the extent to which said electrical storage deviceis being charged with said electrical current, and means for generatinga signal whenever the extent to which said electrical storage device isbeing charged with said electrical current falls below a specifiedvalue. There is further provided such an implantable, rechargeableassembly wherein the implantable device therein is a cardiac assistdevice. There is further provided such an implantable, rechargeableassembly wherein the implantable device therein is a drug deliverydevice. There is further provided such an implantable, rechargeableassembly wherein the implantable device therein is a deep brainstimulation device.

In accordance with this invention, there is further provided animplantable, rechargeable assembly comprised of an implantable devicedisposed within a living organism, a line for connecting saidimplantable device to an electrical storage device, a thermoelectriccharging assembly, a line for operatively connecting said thermoelectriccharging assembly to said electrical storage device, wherein saidthermoelectric charging assembly is comprised of means for transferringthermal energy between said living organism and a thermoelectric module,means for generating an electrical current from said thermal energy,means for charging said electrical storage device with said electricalcurrent, means for determining the extent to which said electricalstorage device is being charged with said electrical current, and meansfor generating a signal whenever the extent to which said electricalstorage device is being charged with said electrical current falls belowa specified value, and a line for operatively connecting said means fordetermining the extent to which said electrical storage device is beingcharged with said electrical current to said electrical storage device.

In accordance with this invention, there is further provided a methodfor increasing the thermal gradient that is present at an implantablepower device implanted in a living organism, said power devicecomprising an electrical storage device and a thermoelectric modulehaving a first surface at a first temperature and a second surface at asecond temperature, wherein said method comprises the steps ofmonitoring at least one condition that may indicate the necessity ofincreasing said thermal gradient; performing a decision to increase saidthermal gradient based upon said monitoring of said at least onecondition; and performing at least one action to cause a change intemperature of at least one of said first surface and said secondsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by reference to the specification and tothe following drawings, in which like numerals refer to like elements,and in which:

FIG. 1 is a schematic diagram of a preferred of one preferred biothermalpower system of this invention;

FIG. 2 is a polarity reversing device that may be used in the powersystem of FIG. 1;

FIG. 3 is a diagrammatic representation of the efficacy of the device ofthis invention at different ambient temperatures;

FIG. 4 is a schematic of one preferred device of this invention;

FIG. 5 is a schematic of a second preferred device of this invention;

FIG. 6 is a schematic of third preferred device of the invention;

FIG. 7 is a flowchart depicting a method of the present invention for amethod for increasing the thermal gradient that is present at a powerdevice of the present invention.

The present invention will be described in connection with a preferredembodiment, however, it will be understood that there is no intent tolimit the invention to the embodiment described. On the contrary, theintent is to cover all alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a preferred biothermal power system 10. Inthe preferred embodiment depicted in FIG. 1, device 10 works on theSeebeck effect.

The Seebeck effect is the development of a voltage due to differences intemperature between two junctions of dissimilar metals. Reference may behad, e.g., to U.S. Pat. No. 5,565,763 (thermoelectric method andapparatus); U.S. Pat. No. 5,507,879 (sensor using thermoelectricmaterials); U.S. Pat. No. 4,019,364 (method of testing welds by usingthe Seebeck effect); U.S. Pat. No. 3,648,152 (Seebeck effectcompensation); U.S. Pat. No. 6,207,886 (Skutterudite thermoelectricmaterial); U.S. Pat. Nos. 6,078,183; 5,952,837; 5,869,892; 5,784,401;5,708,371; 5,491,452 (Peltier element as series noise clamp); U.S. Pat.Nos. 5,446,437; 5,439,528 (laminated thermo element); U.S. Pat. Nos.5,241,828; 5,073,758; 4,938,244; 4,505,427; 4,095,998 (thermoelectricvoltage generator); U.S. Pat. No. 4,026,726, and the like. The entiredisclosure of each of these United States patents is hereby incorporatedby reference into this specification.

Referring again to FIG. 1, thermoelectric module 12 is connected to andprovides electrical current to control circuit 14 by means of leads 16.Control circuit 14, in turn, is operatively connected to a voltageregulator 18. The voltage regulator 18 provides direct current tobattery 20, which, in turn, provides power to the implantable device 22.In the preferred embodiment depicted in FIG. 1, implantable device 22 iscapable of providing stimulation to an organ (such as a human heart) 24via leads 26. The leads 26 may either be electrical leads, and/oroptical leads.

The implantable device 22 also is capable of providing material (suchas, e.g., a therapeutic or stimulatory drug, or a hormone) via conduit28 to one or more sites in a living organism (not shown). Thus, e.g.,such conduit 28 may be caused to deliver an irritant that will causetissue within said living organism to increase its local temperature.Alternatively, or additionally, when a patient becomes aware that thetemperature differential used to power device 10 is not sufficient, hecan topically apply some of the irritant material to some of his tissue.

Power monitor 30 is operatively connected to battery 20 and, by means ofone or more suitable sensors, detects the power level of such battery20. The power monitor 30 is also operatively connected to theimplantable device 22. In one preferred embodiment, the implantabledevice 22 is capable of providing a warning to either the patient and/orhis physician whenever the power being furnished to such device isinadequate. By way of illustration, one may use the warning systemdepicted in U.S. Pat. No. 6,067,473 that uses audible soundcommunication to provide warnings of low battery life; the entiredisclosure of such United States patent is hereby incorporated byreference into this specification. Alternatively, or additionally, onemay use the battery-monitoring device of U.S. Pat. No. 6,108,579, whichprovides a display of the remaining life of the battery; the entiredisclosure of this United States patent is hereby incorporated byreference into this specification.

The battery 20 preferably is a rechargeable battery. Alternatively, oradditionally, other electrical storage devices also may be used. Thus,e.g., one may use a capacitive storage device constructed of carbon orother nanomaterials, or a hybrid device. Such devices are disclosede.g., in U.S. Pat. No. 6,252,762 (“Rechargeable hybridbattery/supercapacitor system”); U.S. Pat. No. 5,993,996 (“Carbonsupercapacitor electrode materials”); U.S. Pat. No. 6,631,072 (“Chargestorage device”); and U.S. Pat. No. 5,742,471 (“Nanostructure multilayerdielectric materials for capacitors and insulators”). The disclosures ofeach of these United States patents are incorporated herein byreference.

In a further embodiment, system 10 is provided without battery 20, andinstead, system 10 is operatively connected to a battery or otherelectrical storage device in a second implanted device such as e.g., apacemaker. Referring to FIG. 1, second implanted device 322 comprisesbattery 320 or other electrical storage device 320. Power system 10comprises primary device 11, which is provided without battery 20 otherelectrical storage device 20. Instead, battery 320 of second implanteddevice 322 is connected to power monitor 30, to implantable device 22,and to voltage regulator 18 via lines 31, 23, and 19, respectively. (Asused herein, a “line” is a strand of material that conducts electricalcurrent, such as e.g., a metal wire.) In this manner, the energygenerated by system 10 is stored in battery 320, and subsequently usedby implanted devices 22 and 322.

Referring again to FIG. 1, and in the preferred embodiment depictedtherein, it will be seen that electrical leads 16 first communicate withpolarity reverser 40 prior to the time they are connected with thecontrol circuit 14. As will be apparent, if the temperature (T_(c)) onsurface 42 of thermoelectric module 12 is higher than the temperature(T_(h)) on surface 44 of the thermoelectric module 12, then the polarityof the electric current produced in such a situation will be the reverseof the situation than when the temperature T_(c) is lower thantemperature T_(h). Absent a polarity reversal device, electrical currentwill not effectively flow into the control circuit under all conditionsof temperature.

The polarity reversal device 40 may be any of the means for reversingpolarity known to those skilled in the art. Reference may be had, e.g.,to U.S. Pat. Nos. 6,232,907; 3,821,621; 4,422,146; 3,623,817, and thelike. The entire disclosure of each of these United States patents ishereby incorporated by reference into this specification.

FIG. 2 is a schematic diagram of one preferred polarity reversal device50 that may be used. Referring to FIG. 2, when lead 52 is negative,electrons flow to point 54, and then through diode 56, and then throughline 58. When, however, the polarity is reversed due to a change in thetemperatures of surfaces 42 and 44 (see FIG. 1), electrons will flowthrough line 60, through to point 62, through diode 64, and then throughline 58. Thus, regardless of whether T_(c) is higher or lower thanT_(h), the temperature difference between them will cause powergeneration. This feature is especially advantageous when the ambientconditions are fluctuating and/or when the body temperature near thedevice 10 is especially low or high.

FIG. 3 is graph of the multiple situations that may occur as T_(c) andT_(h) vary. Referring to FIG. 3, and in zone 70, the body temperature 72is substantially higher than the skin surface temperature 74. Because ofthis positive temperature differential, and referring to FIG. 2,electrons will flow through line 52, to point 54, through diode 56, andthen through line 58. Furthermore, inasmuch as the temperaturedifferential in zone 70 is generally greater than 2 degrees Celsius, aplentiful supply of electrical energy will be produced and supplied tothe control circuit 14 (see FIG. 1).

By comparison, and referring again to FIG. 3, in zone 76 the skinsurface temperature 74 is substantially higher than the body temperature72. Because of this negative temperature differential, electrons willflow through line 60, to point 62, through diode 64, and then to line 58(see FIG. 2). Again, because the temperature differential in this zoneis generally greater than 2 degree Celsius, a generous supply ofelectrons will flow to control circuit 14.

In intermediate zones 78 and 80, however, the temperature differentialmay only be 1 degree Celsius. However, this one-degree positive ornegative temperature differential still is sufficient to provideadequate electron flow to control circuit 14.

However, in zone 82, the temperature differential between the surfaces44 and 42 of module 12 often are less than 1 degree Celsius. In thiszone, the current flow is often inadequate to power the implantabledevice 22. In this case, battery 20 will, over sufficient time, tend todischarge.

In zones 70, 76, 78, and 80, the device 10 will continually chargebattery 20 through means of, e.g., voltage regulator 18; and the amountof electrical charge imparted to battery 20 will exceed the drain onsuch battery caused by implanted device 22.

However, in zone 82, the amount of electrical charge imparted to battery20 will be less than the drain on such battery caused by the needs ofimplanted device 22. If such a situation persists for a long period oftime, the battery 20 will become discharged and have to be replaced witha new battery.

In order to avoid the need to frequently subject a patient to surgery,the device 10 of this invention substantially extends battery life. Onemeans of so doing is to provide a warning to the patient whenever thezone 82 conditions occur for a sustained period of time.

Referring again to FIG. 1, a temperature sensor 41 senses thetemperature outside the living organism, preferably at or through thesurface of the organism's skin. This information is conveyed, via lines43, to power monitor 30.

A second temperature sensor, sensor 45, senses the temperature of theliving organism and provides such information, via lines 47, to thepower monitor 30. Thus, at all times, the power monitor can determinethe difference between the temperature on surfaces 42 and 44, thedirection of such temperature difference, and the amount of time suchtemperature difference has existed.

Power monitor 30 preferably is comprised of means for measuring the rateof current flow into battery 20. In the preferred embodiment depicted inFIG. 1, this information is provided to power monitor 30 by voltageregulator 18. Furthermore, because the power monitor 30 is connected tothe battery 20, it also is continually aware of the charge status of thebattery 20.

Armed with this information, and preferably using an algorithm that maybe periodically modified as necessary upon command from implantabledevice 22, the power monitor can cause the implantable device 22 to emita warning signal. Implantable devices capable of emitting warningsignals have been described elsewhere in this specification.

When such a warning signal has been produced by the implantable device,the patient then has the opportunity to increase the temperaturedifferential between surfaces 44 and 46. He may so do by either puttingon more clothing, taking off some clothing, and/or moving to a warmer orcooler environment.

Referring again to FIG. 1, and in the preferred embodiment depictedtherein, it is preferred that thermoelectric module 12 define “ . . . ahot side and a cold side, said module comprising: A) a plurality ofP-type thermoelectric elements, B) a plurality of N-type thermoelectricelements, said P-type elements and said N-type elements being arrangedin an array and insulated from each other with self adhering polyimidefilm, C) a plurality of contacts on said cold side and said hot sideconnecting said elements in an electric circuit.” This particular moduleis described and claimed in U.S. Pat. No. 6,207,887; the entiredisclosure of this United States patent is hereby incorporated byreference into this specification.

One may use thermoelectric modules, similar to those disclosed in U.S.Pat. No. 6,207,887, but which use different separator elements betweenthe P-type and the N-type elements. Thus, e.g., one may utilizeepoxy-impregnated paper isolators; see, e.g., U.S. Pat. Nos. 3,780,425and 3,781,176, the entire disclosures of each of which is herebyincorporated by reference into this specification.

In one embodiment, a sufficient number of such P-type and N-typeelements of such U.S. Pat. No. 6,207,887 are utilized to provide adevice 10 that, with a temperature differential between T_(h) and T_(c)of only 2 degree Celsius, will produce at least 50 microwatts ofelectrical power at a voltage of from about 0.3 to about 0.5 volts d.c.In another embodiment, the device produces at least 100 microwatts ofpower at such voltage of from 0.3 to about 0.5 volts when presented witha temperature differential of 1 degree Celsius.

In one embodiment, illustrated in FIG. 1, the thermoelectric module 12will have a substantially square shape, a length of from about 1.3 toabout 1.7 inches, and a thickness of from about 0.2 to about 0.3 inches.

Referring again to FIG. 1, and in the preferred embodiment depictedtherein, it will be seen that thermoelectric module 12 is preferablycomprised of a multiplicity of n-doped/p-doped thermocouple pairs(34/36) preferably electrically arranged in series and sandwichedbetween ceramic plates 38 and 40.

FIG. 4 is a schematic representation of the device 10 from which someunnecessary detail has been omitted for the sake of simplicity ofrepresentation. In the preferred embodiment depicted therein, the device10 utilizes the thermoelectric module 12 described in FIG. 1, generatingelectrical power from the temperature gradient between the bodycenterline 90 and the surface of the skin 92 via the Seebeck Effect. Inmost cases the body core temperature is higher that the skin surfacetemperature, but even in cases where this relationship is temporarilyreversed, control circuit 14 (see FIG. 1) will reverse polarity of itsvoltage regulator so that battery charging will continue.

Referring again to FIG. 4, implant device 10 comprises a primary case 94that houses the primary device. In the embodiment depicted, primarydevice 11 is comprised of a controller circuit 14 (not shown, but seeFIG. 1), a voltage regulator 18 (not shown, but see FIG. 1), a powermonitor 30 (see FIG. 1), a battery 20 (see FIG. 1), a polarity reversalunit 40 (see FIG. 1), and an implantable device 22 (see FIG. 1).

In the embodiment depicted, case 94 provides a hermetic seal around allcomponents, provides for passage of lead wires without violatinghermetic seal, and further has specific thermal characteristics. In thisembodiment, case 94 is preferably formed of a titanium/silver bilayer,wherein the titanium provides a robust and biocompatible outer layer,and the silver provides a highly thermally conductive inner layer.

As will be apparent, a variety of combinations of other materials,thickness, and fabrication methods may be used to derive the desiredcombination of strength, magnetic impermeability, biocompatibility, andhigh thermal conductivity.

Referring again to FIG. 4, the thermal conversion module 12 ispreferably disposed immediately against the device case 94 and is inintimate thermal contact therewith, either by silver solder, brazing,conductive grease, or other means. Electrical leads from module 12preferably pass through case 94 via hermetic means. Conductive plate 96is preferably affixed in similar manner to the opposite surface ofmodule 12. Ceramic insulating seal 98 surrounds module 12 and providesthermal isolation between case 94 and conductive plate 96; it and alsoprovides a hermetic seal surrounding the module 12 to protect it fromthe body environment. Thus, ceramic insulating seal 98 should preferablybe made from a specialized material.

One of such specialized ceramic insulating materials is disclosed inU.S. Pat. No. 5,403,792, which describes a low thermal conductivityceramic and process for producing the same. This material comprises asialon (Si—Al—O—N) in combination with one or more of elements La, Dy,Ce, Hf, and Zr. This material is highly rigid, impermeable, and has lowthermal conductivity. The entire disclosure of this United States patentis hereby incorporated by reference into this specification.

By way of further illustration, U.S. Pat. No. 6,015,630 discloses aceramic thermal barrier coating disposed on the metal substrate or onthe optional metallic bond coat, wherein the YAG-based ceramic thermalbarrier coating is selected from the group consisting of Y₃ ^(C) Al₂^(A) Al₃ ^(D) O₁₂, wherein C, A, and D are sites on the crystalstructure, and further wherein all or part of the Al³+ on the A sites, Dsites, or A and D sites are substituted in an amount effective toprovide the YAG-based ceramic thermal barrier coating with a thermalconductivity less than or equal to about 3 Wm⁻¹ K⁻¹ at about 1000° C.,an oxygen diffusivity less than or equal to about 10⁻¹⁵ m² s⁻¹ at about1000° C., a thermal coefficient of expansion greater than or equal toabout 9×10⁻⁶° C.⁻¹, a maximum temperature capability greater than orequal to about 1400° C., a hardness greater than or equal to about 14GPa, an elastic modulus less than or equal to about 280 GPa, or adensity less than or equal to about 6.4 gcm⁻³. The entire disclosure ofthis United States patent is hereby incorporated by reference into thisspecification.

Referring once again to FIG. 4, and in the preferred embodiment depictedtherein, a layer of conformable gel or polymer 100 preferably isdisposed immediately above device case 94, and a conductive sealingmembrane 102 serves to contain polymer/gel 100. In this preferredembodiment, the aforementioned materials provide a highly thermallyconductive path from device case 94 to the skin surface 92 which isimmediately above the sealing membrane 102 but which also provides forhigh degree of comfort for the patient. Materials for polymer/gel 100and membrane 102 are preferably chosen to be biocompatible and also tohave the appropriate flexibility and thermal conductivity.

By way of yet further illustration, U.S. Pat. No. 6,255,376 discloses athermally conductive compound that comprises 15 to 60 volume percent ofthermoplastic carrier resin consisting of a copolymer of a plasticizerwith ethylene or of a polymer of the plasticizer, polyethylene and thecopolymer, 40 to 85 volume percent of thermally conductive fillerparticles dispersed in the carrier resin, and 0.5 to 5 weight percent(for the filler particles) of a dispersing agent having (a) hydrophilicgroup(s) and (a) hydrophobic group(s). The thermally conductive compoundhas a high thermal conductivity and a plasticity in the range oftemperatures of −40 to 50° Celsius. The entire disclosure of this UnitedStates patent is hereby incorporated by reference into thisspecification.

Likewise, U.S. Pat. No. 6,160,042 discloses a method for forming a lowviscosity high thermal conductivity polymer composite containingparticles of hexagonal boron nitride comprising the steps of: (a)treating the surface of the hexagonal boron nitride particles with1,4-phenylene diisocyanate, (b) thereafter reacting the thus-treatedboron nitride particles with a compound of the formula H₂ N—X—Y. Andfurther, U.S. Pat. No. 5,900,447 discloses a composition and method forforming a high thermal conductivity polybenzoxazine-based material. Thecomposition comprises at least one benzoxazine resin and a fillermaterial that includes particles of boron nitride in an amountsufficient to establish a thermal conductivity of between about 3 W/mKand 37 W/mK in the polybenzoxazine-based material. These and other likematerials may be used for membrane 102 and polymer 100 shown in FIG. 4.The entire disclosure of U.S. Pat. Nos. 6,160,042 and 5,900,447 arehereby incorporated by reference into this specification.

Referring again to FIG. 4, insulating sheath 104 is preferably aflexible polymeric insulating material that surrounds the primary devicecase 94, further thermally isolating it from its lateral environmentand, thus, encouraging the primary heat flow to be through the thermalconversion module 12, thence through the device case 94, through theconductive polymer 100, conductive membrane 102, and through the skin92. Insulating sheath 104 may be made from a variety of biocompatibleclosed cell foam materials. One such material is disclosed, e.g., inU.S. Pat. No. 5,137,927 as a composite foam of low thermal conductivitycomprises a) 20–80% by volume of silica aerogel particles having a meandiameter of from 0.1 to 20 mm and a density of from 0.08 to 0.40 g/cm³,b) 20–80% by volume of a styrene polymer foam which surrounds theparticles of component a) and binds them to one another and has adensity of from 0.01 to 0.15 g/cm³, and, if desired, c) conventionaladditives in effective amounts. Another such material is disclosed inU.S. Pat. No. 5,532,284 as an improved closed cell polymer foam andfoaming agent involving the use of a halocarbon blowing agent (e.g.,HCFC-22, HCFC-123, HCFC-123a, and HCFC-141b) in combination with aneffective amount of a gas barrier resin (e.g., an ethylene/vinyl acetatecopolymer, ethylene/acrylic ester copolymer or acrylic ester polymer)uniformly dispersed in the continuous polymeric phase. The presence ofthe gas barrier resin is shown to significantly reduce the escape ofblowing agent from and/or entry of air into the foam resulting in lowthermal conductivity over a longer period of time and improved thermalinsulation value. The entire disclosure of these United States patentsis hereby incorporated by reference into this specification.

FIG. 5 is a schematic design of a second preferred device 110. In theembodiment depicted, the temperature gradient (T_(H)–T_(C)) between thebody core 90 and the surface of the skin 92 provides for electricalpower generation via the Seebeck Effect. Many of the components andmaterials are similar to those corresponding components and materialsdescribed in FIG. 4; however it will be noted that in this embodimentthe conductive polymer/gel 100 and the conductive membrane 102 have beeneliminated, such that the metallic case material 94 will be in directcontact with the interior surface of the skin 92. In addition, thedevice 110 of this embodiment has extensions of insulating sheath 104lateral to device case 94 and immediately under the skin 92, therebyproviding further thermal insulation around the power generation meansand thus improving the thermal efficiency of power conversion. It willalso be noted from FIG. 5 that the portion of the device 110 facinginward into the patient's body core centerline 90 has been extended inorder to further enhance thermal performance by contacting a region thathas a slightly higher average temperature. Conductive plate 96 isattached to another conductive member 106, shown here as an elongatedrod, but which may also be a heat pipe, and which is affixed to thermalcontact 108. Conductive plate 96 is also proximate to or in contact withsurface 44 (see FIG. 1) of module 12, such that conductive heat transferbetween surface 44 and plate 96 is rapid. Insulating sheath 104 has beenextended around conductive member 106 along its length and abuttingthermal contact 108 in order to provide consistent thermal isolationfrom the surrounding tissues and body fluids.

Conductive member 106 is oriented substantially in the direction of thetemperature gradient from body core 90 to skin surface 92, therebyconducting heat into or away from plate 96, depending upon whether thedifference between the temperature of body core 90 and skin 92 ispositive or negative. It will be apparent therefore that the range oftemperature of conductive member 106 is outside of the range intemperature defined by the temperature of surface 44 (see FIG. 1) ofmodule 12 and surface 42 (see FIG. 1) of module 12. The geometry of thisdesign is intended to provide for a higher temperature differential(T_(H)−T_(C)) across module 12, thus providing for greater electricalpower generation.

FIG. 6 is a schematic representation of a device 112 that is similar todevices 110 and 10 (see FIGS. 5 and 4), but differs therefrom in that itis oriented in a manner such that it perpendicular to the core 90; forthe sake of simplicity of representation, many of the elements of suchdevice have been omitted. In this embodiment, thermal module 12 ispreferably sandwiched between a hot plate 114 (that is in immediatecontact with skin surface 92) and a cold plate 116 (that extendsinwardly toward the body core 90). Hot plate 114 and cold plate 116 thusextend in substantially opposite directions. Insulation 104 is used, asdescribed hereinabove, to enhance thermal differentials.

Plate 114 is proximate to or in contact with surface 44 (see FIG. 1) ofmodule 12, such that conductive heat transfer between surface 44 andplate 114 is rapid; and plate 116 is proximate to or in contact withsurface 42 (see FIG. 1) of module 12, such that conductive heat transferbetween surface 42 and plate 116 is rapid. It will be apparent that thedesignation of plate 114 as “hot” and plate 116 as “cold” is done forillustrative purposes, and that actions may be taken to either heat orcool skin surface 92 in order to achieve a desired temperaturedifference between surfaces 42 and 44 (see FIG. 1), resulting in powergeneration by module 12 as described previously. Such actions aredescribed subsequently in this specification.

FIGS. 4, 5, and 6 are intended to be indicative of variations that maybe designed to provide a balance between size, comfort, powerefficiency, device reliability, and ease of positioning the implantwithin the body at various desired positions. In a further embodiment,the system 10 of FIG. 1 is configured in a manner that enables at leastthe thermoelectric module 12 thereof to be positioned at or near thesurface of an airway. In this manner, the flow of air through the airwaydue to respiration will provide cooling upon surface 38 of thethermoelectric module, and since such surface will likely be maintainedin a wet condition, the temperature of such surface 38 will be furtherreduced by evaporative cooling.

Other design features such as electrical or optical connections will beobvious to those skilled in the art, and it will also be obvious tothose skilled in the art that the biothermal power generation processdescribed herein may be scaled either up or down in size to suit powerrequirements of specific implantable devices. Any of the aforementionedchanges may be made in the apparatus without departing from the scope ofthe invention as defined in the claims.

As was previously described in this specification, when the conditionexists as shown in zone 82 of FIG. 3, there may not be sufficientthermal gradient present for the device of the present invention toproduce sufficient electrical power for the intended use. In a furtherembodiment of this invention, there is provided a method for determiningthe necessity of increasing the thermal gradient that is present at thedevice, and also for increasing the thermal gradient that is present atthe device if necessary, in order to increase the power generatedthereby. In various embodiments, the method is performed manually orautomatically by electrical stimulation of tissue, performed manually bythe living body (i.e. the person or “patient”) in which the device isimplanted or by another person, or performed manually or automaticallyby chemical stimulation (irritation) of tissue.

FIG. 7 is a flowchart of a generalized method to increase the thermalgradient that is present at a power device of the present invention.Referring to FIG. 7, method 200 comprises the step 202 of the ongoingmonitoring of conditions that may indicate the necessity of increasingthe thermal gradient that is present at the device. Referring also toFIG. 1, such conditions may include the status of the battery 20,capacitive device, or other electrical energy storage device within orexternal to the power system 10; the temperature difference(T_(h)−T_(c)) or gradient present at the thermoelectric module 12 of thepower device; and the expected energy demand by the device(s) 22 poweredby the system 10. Monitoring step 202 is preferably performed by powermonitor 30, the capabilities of which have been previously described inthis specification.

Referring again to FIG. 7, power monitor 30 is further provided with thecapability to perform decision step 204 on a substantially continuousbasis. If system and ambient conditions are acceptable (“YES”),monitoring step 202 continues. If system and ambient conditions areunacceptable (“NO”), power monitor 30 triggers corrective action. Suchcorrective action may be the provision of an alarm to the patient orother person to manually perform corrective steps, or such correctiveaction may be the automatic execution of corrective steps.

An example of one unacceptable system condition is where the storagedevice 20 is below a desired threshold value of total energy content. Anexample of another unacceptable system condition is where the storagedevice 20 is discharging at an unacceptably high rate, i.e. the powerbeing delivered in from module 12 is so much less than the power beingconsumed by implantable device 22, that the storage device will becomedepleted in an unacceptably short time if corrective action is nottaken. An example of an unacceptable ambient condition is where theabsolute value of the temperature difference between surface 42 andsurface 44 of thermoelectric module 12 (i.e. |T_(h)−T_(c)|) hasdecreased below a threshold value, such that thermoelectric module isnot producing a sufficient amount of power to operate implantable deviceand charge battery 20. Such a condition has been previously described byway of example as zone 82 in FIG. 3.

In one embodiment, such corrective action is via path 210, comprised ofthe step 212 of delivering electrical stimulation to the tissue(s) thatare proximate to or in contact with system 10, and in particular,thermoelectric module 12 of system 10. In one embodiment, electricalleads or contacts (not shown) are provided in contact with such tissues,and electrical impulses are delivered to such tissues therethrough. Suchelectrical impulses may be used to stimulate tissue such as e.g., muscletissue, thereby causing such tissue to intermittently contract andrelax. Such contraction and relaxation will increase the localtemperature of such tissue. The proper selection of the location oftissue to be so stimulated will result in an increase in the temperaturedifference (T_(h)−T_(c)) proximate to thermoelectric module 12 system10. In the preferred embodiment, such stimulation is providedautomatically in response to decision 204. However, in an alternativeembodiment, an alarm may be provided to the patient or another person toprovide such stimulation.

In another embodiment, in response to a “NO” answer in decision step204, corrective action is via path 220, comprised of a first alarm step222, in which the patient or other person is alerted of the need ofcorrective action required. Such corrective action may include the step224 of delivering energy to the tissue proximate to or in contact withthermoelectric module 12 through mechanical heat transfer means, or thestep 226 of delivering energy through chemical means, or both.

In step 224, such mechanical heat transfer means include heat transferby conductive means, heat transfer by convective means, and heattransfer by radiative means, and combinations thereof. In general, theeffect of such mechanical heat transfer means is preferably applied tothe skin of the patient in order to produce the desired temperaturegradient, and may include either a heating or a cooling of such skin.

For example, heat transfer by convective means may be achieved by thepatient moving close to a convective air conditioner or space heaterthat is discharging a stream of conditioned air, or by using a hand-heldheating device such as a blow dryer to heat the skin. Heat transfer byconductive means may be achieved by the application of ice or other coldobjects to the skin, or by contact with an electrical heating pad, or bythe wearing of a chemically generated heat source intended for suchpurposes, such as e.g., the ThermaCare® heat wrap that is sold by theProctor and Gamble Corporation of Cincinnati, Ohio. Heat transfer byradiative means may be achieved by the patient moving close to aradiative heater that is radiating heat energy, such as an electricallypowered infrared heater, a wood stove, and the like.

In the embodiment comprising step 226 of delivering energy throughchemical means, such chemical means may include e.g., the topicalapplication of some an irritant material to the patient's tissue by thepatient or by another. Among the suitable irritants are those chemicalsubstances such as menthol, which increase the blood circulation nearthe surface of the skin, but are not toxic to the skin or other tissues.A commercial example of such an irritant is “FLEXALL 454”, distributedby Chattam, Inc. of Chattanooga, Tenn. Such irritants result in anincrease in temperature at or near the surface of the skin.

In another embodiment, in response to a “NO” answer in decision step204, corrective action is via path 230 comprised of step 232, in whichchemical stimulation is delivered to the tissue proximate to or incontact with thermoelectric module 12 of FIG. 1. In the preferredembodiment, such chemical stimulation is delivered by implantable device22, which may include a drug or chemical delivery pump or other deliverymeans (not shown). The substance used in such chemical stimulation mayinclude an irritant or stimulant that results in an increase incirculation in the tissue, but is not toxic to bodily tissues.

In each of the alternate pathways 210, 220, or 230, the monitoring 202of critical conditions described previously continues or is performed ata suitable frequency, such that decision 254 can be made. In thecircumstance where the corrective action has made conditions acceptable,(“YES”), the corrective action is terminated, and monitoring 202continues. In the circumstance where the corrective action has not madeconditions acceptable, (“NO”), several options are available. In oneoption, the treatment of the selected path (210, 220, or 230) may becontinued. In other options, one or more of the paths 210, 220, or 230may be selected alternatively or additionally to the originally chosenpath 210, 220, or 230. In the event that the conditions are notacceptable, and none of paths 210, 220, or 230 appear to be acceptable,step 260 is performed as a last resort, wherein external intervention isperformed on system 10. Such intervention may include invasive surgeryto repair or replace system 10.

FIG. 7 is intended to be indicative of a general method to increase thethermal gradient that is present at a power device of the presentinvention. It will be apparent that many additional variations may bemade in the general method disclosed without departing from the scope ofthe invention as defined in the claims.

1. A method for increasing the thermal gradient that is present at animplantable power device implanted in a living organism, said powerdevice comprising an electrical storage device and a thermoelectricmodule having a first surface at a first temperature and a secondsurface at a second temperature, wherein said method comprises the stepsof: a. monitoring at least one condition that may indicate the necessityof increasing said thermal gradient; b. performing a decision toincrease said thermal gradient based upon said monitoring of said at(east one condition; and c. performing at least one action to cause achange in temperature of at least one of said first surface and saidsecond surface.
 2. The method as recited in claim 1, wherein said atleast one condition is the absolute value of the difference between saidfirst temperature and said second temperature.
 3. The method as recitedin claim 1, wherein said at least one condition is the total energycontent of said electrical storage device.
 4. The method as recited inclaim 1, wherein said at least one condition is the rate of discharge ofsaid electrical storage device.
 5. The method as recited in claim 1,wherein said at least one action to pause a change in temperature of atleast one of said first surface and said second surface is delivery ofelectrical stimulation to a tissue of said living organism.
 6. Themethod as recited in claim 5, wherein said delivery of electricalstimulation to said tissue of said living organism is automatic.
 7. Themethod as recited in claim 1, wherein said at least one action to causea change in temperature of at least one of said first surface and saidsecond surface is delivery of energy to a tissue of said living organismby mechanical heat transfer means.
 8. The method as recited in claim 7,wherein said delivery of energy to a tissue of said living organism bymechanical heat transfer means is performed by means selected from thegroup consisting of conductive heat transfer means, convective heattransfer means, radiative heat transfer means, and combinations thereof.9. The method as recited in claim 1, wherein said at least one action tocause a change in temperature of at least one of said first surface andsaid second surface is delivery of energy to a tissue of said livingorganism by chemical means.
 10. The method as recited in claim 1,wherein said implantable power device further comprises chemicaldelivery means, and wherein said at least one action to cause a changein temperature of at least one of said first surface and said secondsurface is delivery of a chemical by said chemical delivery means to atissue of said living organism.
 11. The method as recited in claim 1,further comprising the step of providing an alarm to said livingorganism to perform said at least one action to cause a change intemperature of at least one of said first surface and said secondsurface.
 12. The method as recited in claim 1, further comprising thestep of performing a decision to continue said at least one action tocause a change in temperature of at least one of said first surface andsaid second surface.
 13. The method as recited in claim 1, furthercomprising the step of performing a decision to discontinue said atleast one action to cause a change in temperature of at least one ofsaid first surface and said second surface.